An arc-welding method is known, which comprises, when welding objects of welding together in the longitudinal direction of a groove formed between the objects of welding by the arc-welding method using a consumable or non-consumable welding electrode, reciprocally moving a welding torch, following the face of the groove, in the width direction of said groove, and, at the same time, continuously moving said torch in the longitudinal direction of said groove, to automatically weld the objects of welding together in the longitudinal direction of the groove.
In order to weld the objects of welding together by the above-mentioned method, it is necessary to reciprocally move the welding torch, following the face of the groove, in the width direction of and in the vertical direction toward the groove, and reverse the direction of movement of the welding torch at a prescribed position relative to the groove while maintaining the distance between the tip of the welding electrode and the groove at a prescribed distance. In order to cause the welding torch to follow the face of the groove, it is necessary to detect the position of the tip of the welding electrode relative to the groove.
For the purpose of detecting the position of the tip of the welding electrode relative to the groove, the conventional practice has been to employ a contact type detecting sensor which detects the position of the welding electrode relative to the groove, by providing a roller as a follower near the welding electrode and causing this roller to be in contact with the face of the groove, with the use of a differential transformer, a potentiometer or a limit switch, or to employ a non-contact type detecting sensor which detects the welding electrode relative to the groove, by an electromagnetic or optical position detector provided near the welding electrode, with the use of magnetism or light.
In any of the above-mentioned conventional detecting methods, there is a certain distance between the detecting position by the above-mentioned sensor and the position of the welding electrode, since the position of the welding electrode is detected by the sensor provided at a certain distance apart from the welding electrode. When controlling the position of the welding electrode, therefore, it was necessary to set a time-lag depending upon the aforementioned distance to correct the result detected by the sensor, and this has led to a complicated control device and to the problem of the impossibility of achieving a high accuracy in controlling.
As a welding method to solve these problems, therefore, we proposed an arc-welding method as disclosed in Japanese Patent Provisional Publication No. 19,445/79 dated Feb. 14, 1979 (hereinafter referred to as the "prior art"). The prior art is a method which comprises, without separately providing a sensor for detecting the position of the welding electrode, detecting the position of the welding electrode by means of the arc itself produced between the tip of the welding electrode and the groove, and causing the tip of the welding electrode to follow the face of the groove by controlling the position of the welding electrode on the basis of the result of detection, as summarized as follows:
The method of the prior art comprises: directing a welding torch substantially vertically toward a groove formed between objects of welding; directing a welding electrode through said torch toward said groove; feeding a welding current to said electrode to produce an arc between the tip of said electrode and said groove to weld the objects of welding together by means of the arc heat; moving said torch in the width direction of the groove; continuously detecting one of arc voltage and arc current of said arc; calculating the deviation of the thus detected value from the value of one of previously set arc voltage and arc current; moving the torch vertically up and down so that the deviation becomes null, thereby maintaining the distance between the tip of the electrode and the groove at a prescribed distance during the movement of said torch in the width direction of said groove; continuously detecting the vertical position of the torch as a value of voltage indicated by a potentiometer for each movement of said torch in the width direction of the groove; reversing the direction of the movement of said torch in the width direction of the groove at the moment when the value of voltage thus detected agrees with the previously set value of voltage; repeating the movement of the torch in the width direction of the groove, the movement of the torch in the vertical direction, and the reversal of the direction of the movement of the torch in the width direction of the groove; on the other hand, continuously moving the torch in the longitudinal direction of the groove; thereby reciprocally moving the torch in the width direction of the groove following the face of the groove while maintaining the distance between the tip of the electrode and the groove at said prescribed distance, and, at the same time, moving the torch in the longitudinal direction of the groove so as to weld the objects of welding in the longitudinal direction of the groove.
According to the prior art presented above, since it is possible to detect the position of the welding electrode by the arc itself and carry out welding by causing the welding electrode to follow the groove face on the basis of the result of this detection, it is not necessary to separately provide a sensor for detecting the position of the welding electrode, and it is possible to control the position of the welding electrode at a high accuracy.
However, the above-mentioned prior art was found to involve the following problems. Upward or downward inclinations or wavy deformations are often existent on the bottom face of the groove formed between the objects of welding. According to the above-mentioned prior art, since the reversal of the reciprocating torch in the width direction of the groove is conducted at the moment when the detected value of voltage indicating the position of the torch, i.e, the height of the torch agrees with a prescribed value of voltage representing the reversing position of the torch, i.e., the reversing height of the torch determined on the basis of a height of rails on which a carriage of the torch travels, the torch would be reversed always at the above-mentioned reversing height of the torch, irrespective of the presence of the upward or downward inclinations or the wavy deformations appearing on the bottom face of the groove, i.e., not at a certain height from the bottom face of the groove. As a result, the height of the formed bead from the bottom face of the groove becomes non-uniform, thus resulting in a defective penetration and production of welding defects, and leading to the impossibility of reversing the torch when the bottom face of the groove inclines upward.